Question

why is carbon monoxide harmfull when inhaled

Answer 1

When Carbon monoxide is inhaled, it displaces the oxygen in our body and forms Carboxy haemoglobin. This Carboxy haemoglobin passage through body parts may effect them a lot. This effect may sometimes be long lasting too. Inhaling large amount of CO at a second may immediately make you unconscious and leads to breathe suffocation. 

Answer 2

Carbon monoxide is harmful when breathed because it replaces oxygen in the RBC'S of blood which effects in supply of sufficient oxygen to the tissues or cells it is very harmful gas 
it will get into hemoglobin of blood and become carboxy haemoglobin which affects our cells
it may lead to suffocation , unconsciousness